1. Field of the Invention
The present invention relates to an electrostatic image developing toner used in a developer for developing electrostatic images in electrophotography, electrostatic recording, electrostatic printing and the like, and also relates to a production method of the toner. More specifically, the present invention relates to an electrostatic image developing toner used in copiers, laser printers, plain paper facsimiles etc. using a direct or indirect electrophotographic developing system, and also relates to a production method of the toner.
2. Description of the Related Art
There has been a strong market demand for achieving high-quality image formation on electrophotographic apparatuses. Accordingly, developments have been made to make toner responsive to the demand for forming high quality images. In order to make toner responsive to achieving high-quality image formation, uniformization of toner particle size is an indispensable prerequisite. When a toner is made to have uniform toner particle size and a sharp particle size distribution, behaviors of individual toner particles are substantially uniformized, resulting in a remarkable improvement in microdot reproducibility.
In color-image formations using a full-color electrophotographic process, generally, three color toners of yellow, magenta and cyan colors, or four color toners of black color toner in addition to the three color toners are mixed to reproduce all colors. Thus, in order to obtain a sharp full-color image excellent in color-reproducibility, it is necessary to smooth the surface of a fixed toner image to some extent so as to reduce light scattering. For this reason, as to image glosses obtained by conventional full-color copiers etc., images having a middle glossiness to a high-glossiness of 10% to 50% have been often formed.
Generally, as a method of fixing a dry-toner image on a recording medium, the contact-heating fixing method is often employed in which a roller or belt having a smooth surface is press-contact with toner while heating the roller or belt. This method has advantages in that it exhibits high-thermal efficiency, enables high-speed fixing and enables color toners to have glossiness and transparency. On the other hand, this method inconveniently causes a so-called offset phenomenon in which a part of a toner image adheres to the surface of a fixing roller and then transferred onto another image, because a surface of a heat-fixing member is made in contact with a molten toner under application of pressure and then they are separated from each other.
With a view to preventing the offset phenomenon, there has been carried out a method of forming a surface layer of a fixing roller with use of a material excellent in releasing property, such as a silicone rubber and a fluorine resin, and further applying a releasing oil, such as a silicone oil, onto the surface layer of the fixing roller. This method is extremely effective in preventing toner-offset.
However, this method requires additionally providing a device for supplying a releasing oil, leasing to upsizing the fixing device and high costs.
With respect to monochrome toners, a method is employed in which a molecular weight distribution of a binder resin is adjusted so as to increase the viscoelasticity of a molten toner and to avoid internal fracture of the molten toner, and further, a releasing agent such as a wax is incorporated into the toner so that no releasing oil is applied to a fixing roller or the amount of oil applied thereto is reduced to a very small amount.
As described above, however, in use of a color toner, there is a need to smooth the surface of a fixed image in order to improve the color reproducibility, and thus to reduce the viscoelasticity of the toner in a molten state. Therefore, color toners can more easily cause the offset phenomenon than monochrome toners, which have less glossiness, and it becomes more difficult to employ oil-less fixing method on a fixing device and to apply a very small amount of oil to a fixing device. When a releasing agent is incorporated into a toner, the adhesive strength of the toner is increased, so that the transferability of the toner to a transfer paper degrades, and further, the releasing agent in the toner contaminates a frictional charging member such as a carrier, which causes degradation in chargeability, resulting in degradation in durability of the toner.
For instance, Japanese Patent Application Laid-Open (JP-A) No. 8-220808 proposes a toner using a linear polyester resin having a softening point of 90° C. to 120° C. and a carnauba wax. Japanese Patent Application Laid-Open (JP-A) No. 9-106105 proposes a toner using resins which are soluble with each other and have a different softening point, and a wax. Japanese Patent Application Laid-Open (JP-A) No. 9-304964 proposes a toner using a polyester resin and a wax having a specified melt-viscosity. Japanese Patent Application Laid-Open (JP-A) No. 10-293425 proposes a toner using a polyester resin having a softening point of 90° C. to 120° C., a rice wax, a carnauba wax and a silicone oil. Furthermore, Japanese Patent Application Laid-Open (JP-A) No. 5-61242 discloses a wax-incorporated toner which is produced by polymerization.
The above-mentioned toners, however, are not toners having sufficient offset resistance while maintaining a moderate glossiness, even used in a fixing method where no releasing oil is applied to a fixing roller or the amount of oil applied thereto is reduced to a very small amount, and excellent in transferability, durability, charging stability against humidity and pulverizability.
The releasing property of toner is greatly affected by the dispersion state of wax in a binder resin. When the wax is mutually soluble in the binder resin, the wax does not exhibit its essential melting capability and thus cannot exhibit its releasing property. The offset resistance of toner can be improved only by allowing the wax to exist as domains in the binder resin.
Accordingly, it is advantageous in terms of improving the offset resistance of toner to use a wax poor in solubility with binder resins. The poorer the mutual solubility between a binder resin and a wax, the larger is the dispersion diameter of the wax domains. When the dispersion diameter of wax is large, the amount ratio of wax residing close to the surface of toner particles is relatively increased, and the wax component easily exude to the surface of the toner, which is beneficial in terms of the offset resistance.
However, when the wax is easily exposed to the surface of the toner, the wax and carrier moves toward the photoconductor, causing filming at the photoconductor. Further, when the wax moves to the carrier and the like, it causes aggregation of toner particles, resulting in degradation in flowability of the toner.
As described above, when the dispersion diameter of the wax is increased so as to increase the amount ratio of wax residing close to the surface of toner particles, the offset resistance is improved, but on the other hand, which brings about new problems of preventing formation of high-quality images, such as filming and aggregation of tone particles.
Note that when wax components are exposed from the surface of the toner in formation of toner particles in an aqueous medium, it causes aggregation of the toner particles, which adversely affects a particle size distribution of the toner.
In order to prevent the adverse affect of the wax exposed from the surface of toner, as mentioned above, a toner having a capsulated structure (capsulate toner) has been proposed. Since this capsulate toner has a core layer composed of wax and a shell layer which covers the core layer and which is composed of a binder resin, wax components are prevented from being exposed from the surface of toner particles. In other words, a capsulate toner can prevent the occurrence of filming and aggregation of toner particles.
However, this capsulate toner has a structure that wax components are tightly covered with the shell layer, and thus this capsulate toner requires much more energy and longer time to allow the wax components exude from the inside of toner than conventional toners containing wax components, so that it is sometimes unable to obtain satisfactory releasing property.